1. Field of the Invention
This invention relates to an electronic device substrate, an electronic device and methods for making the same and, in particular, to an electronic device that, despite having a coreless type structure advantageous to thinning or downsizing, is less likely to have a crack during its production process and has a structure advantageous for external mounting and a method of making the same. Also, this invention relates to an electronic device substrate used for the electronic device and a method of making the same.
2. Description of the Related Art
FIGS. 8A and 8B are a cross sectional view and a top view (mounting surface), respectively, showing a conventional electronic device.
The electronic device 100 comprises: a wiring substrate 102 with a through hole 103 formed therein; a die pad 104 formed on the wiring substrate 102; plural wiring patterns 105 which are formed on the wiring substrate 102 and each have a metal electrode 105a and an internal connecting terminal 105b formed on both ends thereof; an electronic parts 106 mounted and bonded onto the die pad 104 through a conductive paste etc. (not shown); plural external electrode pads 107 formed on the bottom of the wiring substrate 102 and connected to the lower end of the through hole 103; ball-shaped external connecting terminals 108 formed on the external electrode pads 107; bonding wires 109 formed of Au wire etc. and connecting a terminal 106a with the internal connecting terminal 105b; and a sealing resin 110 formed on the wiring substrate 102 to cover the electronic parts 106 and the bonding wires 109.
The wiring substrate 102 is formed of glass epoxy resin, polyimide tape etc.
The through hole 103 is a conductor for electrically connecting the metal electrode 105 with the external electrode pad 107. The die pad 104, the wiring pattern 105 and the external electrode pad 107 are made of a copper foil etc. formed by photochemical etching etc.
The through hole 103, the die pad 104, the wiring pattern 105 and the external electrode pad 107 are provided with copper plating, nickel base plating, gold plating etc.
The electronic device 100 as shown in FIG. 8A is produced such that the electronic parts 106 is mounted on the die pad 104 of the wiring substrate 102, the terminal 106a of the electronic parts 106 is connected through the bonding wires 109 to the internal connecting terminal 105b, and the sealing resin 110 formed of epoxy resin etc. is formed thereon. Typically, in the final step, the ball-shaped external connecting terminal 108 such as a solder ball is attached onto the external electrode pad 107.
Recently, a coreless type electronic device (hereinafter called coreless package) is proposed which does not use a module substrate. For example, JP-A-3-94459 (pages 3-4 and FIG. 1) discloses a coreless package that electronic parts is die-bonded on a base film and wire-bonded to a metal base, and then unnecessary parts of the metal base is removed by etching to expose terminals and a mounting portion.
FIG. 9 shows the structure of the coreless package as disclosed in JP-A-3-94459. The coreless package 120 is constructed such that the wiring substrate 102, as an insulating core substrate, in FIG. 8A is removed, and the back surface of the die pad 104 and the wiring pattern 105 is exposed on the bottom of the package.
JP-A-3-99456 (pages 2-3 and FIG. 1) discloses a coreless package with plural electronic parts elements. The coreless package is constructed such that the plural electronic parts are wire-bonded to a circuit pattern, these are resin-sealed integrally, a protective coat is formed on the bottom of the package, and gold plating for corrosion prevention is formed on the circuit pattern exposed at an opening of the protective coat.
FIGS. 10A to 10E show a method of making the coreless package 120 as shown in FIG. 9.
First, as shown in FIG. 10A, on an insulating transfer film 121 as a core substrate, the die pad 104, and the wiring pattern 105 having the metal electrode 105a and the internal connecting terminal 105b are formed.
Then, as shown in FIG. 10B, the electronic parts 106 is mounted on the die pad 104. Then, as shown in FIG. 10C, the terminal of the electronic parts 106 is connected through the bonding wires 109 to the internal connecting terminal 105b. 
Then, as shown in FIG. 10D, the electronic parts 106 is sealed with the sealing resin 110. Then, by removing the transfer film 121, the coreless package 120 as shown in FIG. 10E can be obtained. This method is generally called transfer method, where the wiring conductor is transferred to the side of the sealing resin 110.
JP-A-9-252014 (paragraphs [0007]-[0010] and FIG. 2) discloses a transfer method for making the coreless package by using a thick base material instead of the transfer film. This method is conducted such that a metal foil is pasted to the base material, an electronic parts is mounted on the metal foil and wire-bonded, a resin is sealed thereon, and the sealing resin is separated from the based material.
JP-A-2002-9196 (paragraphs [0016]-[0025] and FIGS. 2-3) discloses a transfer-like method for making the coreless package where the metal base located at the bottom is etched. This method is conducted such that a resist pattern is formed on the metal base as a core substrate, openings are formed in the resist pattern corresponding to a die bonding portion and a bonding portion, the opening is filled with nickel plating and gold plating is formed on the surface of the nickel plating, the resist pattern is removed, an electronic parts is mounted on the die bonding portion, wire bonding is made onto a gold plating film as the bonding portion, a resin is sealed thereon, and the metal base is removed by etching.
Conductors for the die pad, the internal connecting terminal, the wiring pattern, the external connecting electrode etc. are typically formed of a copper foil such as an electrolytic copper foil, a rolled copper foil etc. The copper foil is patterned by photochemical etching to form the die pad, the internal connecting terminal, the wiring pattern, the external connecting electrode etc.
FIG. 11 shows the detailed composition of a transfer film unit including the transfer film 121 as shown in FIG. 10A. The transfer film unit comprises: an adhesive 122 coated on the transfer film 121; the die pad 104 and the wiring pattern 105 formed on the adhesive 122; and a functional plating 123 formed on the surface of the die pad 104 and the wiring pattern 105.
The functional plating 123 is used to facilitate the connection between the terminal of the electronic parts 106 and the metal electrode 105. For example, the functional plating 123 is formed of, as a base plating, an electroless nickel plating or electrical nickel plating, and an electroless gold plating or electrical gold plating formed thereon.
In general, the electrical nickel plating is formed 0.5 to 2.0 μm in thickness depending on heating conditions in the mounting or wire bonding of electronic parts. The base nickel plating serves as a prevention film (or a barrier film) for thermal diffusion of copper into gold plating film. The gold plating is formed as a surface layer since it has high connection reliability in ultrasonic wire bonding. Its thickness is desired to be as thick as possible to enhance the wire-bonding performance, but its optimum thickness is selected 0.1 to 2.0 μm in view of the productivity and manufacturing cost.
However, the conventional electronic devices each have the following problems.
The composition as shown in FIG. 11 has a problem that the gold plating of the functional plating 123 is poor in adhesiveness with the sealing resin 110. Namely, when the gold plating is formed on the surface of the functional plating 123, adhesiveness with the sealing resin deteriorates to lower the reliability of the electronic device since the gold does not create any oxide film with high electro negativity thereon.
The BGA structure as shown in FIG. 8A is advantageous in that the external mounting of the electronic device is rendered easy by the protruding external terminal 108 such as a solder ball. However, the distance between adjacent electrodes has to be more than the diameter of the ball. Further, the total thickness of the electronic device must be increased since the diameter of the ball is added to the thickness of the electronic device.
In producing the coreless package 120 as shown in FIGS. 10A to 10E, the transfer film 121 is removed in the final step. However, the adhesive 122 provided on the transfer film 121 may be left on the back surface of the metal electrode 105a even after separating the transfer film 121. Further, the transfer film 121 may be partially left thereon due to the imperfect separation.
In order to solve this problem, JP-A-2002-9196 discloses a method of connecting the metal base, as a core substrate, and the electrode through a metal with low adhesiveness. However, in this method, if the adhesive force between the transfer film 121 and the wiring pattern 105 is more than that between the sealing resin 110 and the wiring pattern 105, the wiring pattern 105 may be separated from the sealing resin 110 while adhering to the transfer film 121 when the transfer film (=core substrate) 121 is peeled off.
In order to solve this problem, JP-A-2002-9196 discloses a further method that the metal for the wiring pattern is thickened and an extended portion is, at the periphery, provided which extends slightly to the side of the sealing resin. However, this method causes an increase in plating time to have the increased thickness of the wiring pattern. Further, the resist film needs to be removed while keeping the shape of the apprentice (=extended portion). Because of these, it is difficult to shorten the distance between adjacent electrodes. As a result, the size of the electronic device must be increased.
Further, when the metal base is peeled off by applying a mechanical stress, the stress may cause warp or crack in the electronic device. Therefore, such a method is not suitable for the formation of, especially a low-profile electronic device.
Further, as shown in FIG. 9, it is impossible to form the wiring pattern 105 under the electronic parts 106 since the die pad 104 is placed there. Thus, the conventional coreless package designs have the problem that the mounting area of the coreless package 120 cannot be reduced.